In recent years, projection-type liquid crystal displays are widely used as display units for displaying on a big screen. Rear projection-type liquid crystal displays are mainly used for big-screen TVs and front projection-type liquid crystal displays for presentation of personal computer data. A projection-type liquid crystal display has a structure to enlarge and project an image on small liquid crystal elements onto a big screen by using an optical system of projection. A detailed technical description can be found, for embodiment, in Non-Patent Document 1 (big-screen display).
FIG. 1 shows a configuration of a typical projection-type liquid crystal display. Light from a light source 4 is separated into blue (B), green (G), and red (R) components by optical components 5 to 16 and is the light components are guided to corresponding liquid crystal elements 2B, 2G, and 2R, respectively. The liquid crystal elements 2R, 2G, and 2B have incident-side polarizers 1R, 1G, and 1B on the incident side and exit-side polarizers 3R, 3G, and 3B on the exit side respectively. A set of polarizers each having an incident-side polarizer and an exit-side polarizer corresponding to red, green, or blue has a function to selectively allow light passed through the liquid crystal element in a predetermined polarization direction to pass. This function allows lights in three primary colors passed through the liquid crystal elements 2B, 2G, and 2R to become a light intensity modulated image signal. These lights in three primary colors are further synthesized optically by a synthesis prism 17 and further projected onto a screen 19 through a magnifying projector lens system 18.
Polarization characteristics required for polarizing elements include a property that transmits optical signals having a desired polarization plane, while at the same time blocking unnecessary optical signals having a polarization plane perpendicular thereto. That is, a desired property is to have a large transmittance with respect to light having a desired polarization plane and a small transmittance with respect to light having a polarization plane perpendicular thereto.
The ratio of these transmittances is called an extinction ratio and is widely used by those skilled in the art as a performance index representing performance of a polarizing element. Using this index, performance required for polarizing elements applied to a projection-type liquid crystal display can be expressed as having a large transmittance and a large extinction ratio with respect to an optical signal. An industrially usable polarizer is said to preferably have the transmittance of 70% or more with respect to light of the wavelength to be used and the extinction ratio of 10:1, preferably 3000:1 (Patent Document 1).
A social demand for a projection-type liquid crystal display is a demand to realize bigger and clearer images by a smaller device. To realize this demand, a recent technical trend is to apply a light source of a larger quantity of light and to use smaller liquid crystal elements. As a result, light of higher energy density is introduced not only to liquid crystal elements, but also to polarizers placed before and after the liquid crystal elements. Particularly high heat resistance and light resistance are increasingly demanded for polarizers having a function to absorb unnecessary light.
According to principles of polarizers, dichromatic polarizers that selectively absorb light depending on the polarization plane and non-dichromatic polarizers (such as a Brewster polarizer) are known (See Patent Document 2). Dichromatic polarizers have thin elements and do not need any special device to absorb unnecessary light and thus are desired elements for projection-type liquid crystal displays whose miniaturization is particularly demanded.
Currently, dichromatic polarizers realizing practical optical performance in the visible light region are only polarizing films made of organic material. However, polarizers made of organic resin have a fatal defect of low heat resistance (See Patent Document 1).
To rectify the defect, polarizing films made of organic resin are used by sticking polarizing films to a sapphire substrate having a high thermal conductivity (Patent Document 3). However, polarizers stuck to sapphire having an excellent thermal conductivity cannot satisfy technical requirements of higher intensity in recent years, that is, requirements that no degradation of polarizer functions caused by light absorption/heat generation by polarizing elements in a green region with the highest intensity occur. Accordingly, a cooling device including a cooling fan is installed in a projection-type liquid crystal display to protect organic resin films from heat. The cooling device not only is against social needs of miniaturization, but also creates another problem of noise.
As a method to solve this technical problem, an idea of applying polarizing glass applied to elements for optical communication has been proposed (Patent Document 1). However, the wavelength of light used for optical communication is in a far-infrared region and is vastly different from that of visible light and thus, technology of glass polarizers for optical communication cannot be immediately applied to projection-type liquid crystal displays controlling visible light. The invention disclosed in Patent Document 1 does not disclose any technology to provide effective characteristics to glass polarizing elements with respect to light in the visible light region and therefore, it is difficult to realize a projection-type liquid crystal display using glass polarizers by using only this invention.
Here, the technical background of polarizing glass will be briefly described. The polarizing glass is glass characterized in that metallic fine particles having shape anisotropy oriented and dispersed in an optically transparent glass substrate are contained and realizes polarization characteristics by an anisotropic resonance absorption phenomenon of surface plasmons present on the surface of metallic fine particles (See Patent Document 4 and Non-Patent Document 2).
Surface plasmon resonance absorption characteristics of metallic fine particles in Patent Document 4 are cited as FIG. 2. A graph A in FIG. 2 corresponds to surface plasmon resonance absorption by spherical metallic fine particles. Resonance absorption of metallic fine particles having cylindrically stretched shape anisotropy shows different characteristics due to correlations between the polarization plane of incident light and metallic fine particles having shape anisotropy.
When the polarization plane is in parallel with the longitudinal direction of metallic fine particles, characteristics indicated by B are exhibited. It is seen that the wavelength of resonance absorption is shifted to a longer wavelength as compared with the characteristics A. It is known that this resonance absorption wavelength depends on the ratio of a longer diameter to a shorter diameter of metallic fine particles and the resonance absorption wavelength becomes larger as the ratio increases (See Non-Patent Document 2). With respect to light having the polarization plane perpendicular to the longitudinal direction, on the other hand, properties shown by characteristics C are exhibited.
FIG. 2 shows that the glass has polarization characteristics with respect to light near 600 nm. That is, the glass has a small transmittance with respect to light having the polarization plane in parallel with the longitudinal direction of metallic particles due to strong absorption. The transmittance with respect to light having the polarization plane in parallel with the longitudinal direction of metallic particles will be represented as T∥ % below. On the other hand, the glass shows poor absorption of light having the polarization plane perpendicular to the longitudinal direction of metallic particles and therefore, a larger transmittance. The transmittance with respect to light having the polarization plane perpendicular to the longitudinal direction of metallic particles will be represented as T⊥ % below. Polarization characteristics are realized by the mechanism described above. Incidentally, characteristics disclosed in FIG. 2 do not realize characteristics required for projection-type liquid crystal displays, that is, the characteristics that the ratio of the parallel absorption curve B to the perpendicular absorption curve C between 500 nm and 600 nm, namely, the extinction ratio is sufficiently large and the value of parallel absorbance is sufficiently large.
Many technologies have been proposed for polarizing glass and glass polarizers using polarizing glass. Many of these technologies relate to glass polarizers applicable to light in the infrared region (such as Patent Document 5 and Patent Document 6) and no technology applicable to light in the visible light region used in a projection-type liquid crystal display, which is an object of the present invention, is disclosed.
Only a small number of inventions relate to glass polarizers applicable to light in the visible light region. Patent Document 7 discloses a technology to provide polarizing elements effective for light in the visible light region by using characteristics of copper fine particles having shape anisotropy (Disclosed characteristics are cited in FIG. 3). However, as seen in FIG. 3, a large extinction ratio particularly for wavelengths equal to 600 nm or less cannot be realized, that is, the ratios (extinction ratios) of values of parallel transmittance curves D and F to those of transmittance curves C and E perpendicular to the stretch axis are small and also the value of the transmittance C is only 10 to 60%, leading to a conclusion that the polarizing elements do not have practical characteristics.
Patent Document 8 discloses a technology to realize dichromatic absorption with respect to wavelengths in the visible light region. However, characteristics applicable to a projection-type liquid crystal display, which is an object of the present invention, that is, a high transmittance and a high extinction ratio are not specifically and quantitatively described and thus, the technology cannot be considered to be able to realize polarizers. Like Patent Document 8, Patent Document 9 proposes a technology to obtain an effective extinction ratio in the visible light region, but no technology to realize a high transmittance is disclosed.
CODIXX AG offers polarizing glass effective in the visible light region by using a manufacturing technique providing shape anisotropy to silver fine particles by introducing silver ions by diffusion from the glass surface, causing silver fine particles to deposit by heat treatment and stretching the glass (Non-Patent Document 3). However, since the ion diffusion process is generally unstable and concentrations of silver ions are distributed in the thickness direction of the glass, dimensions of generated silver particles tend to be non-uniform. As a result, the ion diffusion process has a weak point of producing fluctuations in characteristics of polarizers. Further, there is a problem that the glass is more likely to break when being stretched because particles to be stretched are solid metallic silver particles for which more stretching tension is needed than silver halide particles stretched in a liquid drop state.
A different manufacturing method is used for infrared glass polarizers for communication industrially widely used. That is, as described in Patent Document 4 and Patent Document 5, a manufacturing method by which silver fine particles are produced by temporarily causing silver halide to deposit and then reducing silver halide is adopted. However, polarizers manufactured by this manufacturing method do not exhibit practical performance that can be used in visible light region (Patent Document 5). For embodiment, FIG. 1 (cited herein as FIG. 4) and paragraph [0022] of the specification of Patent Document 5 describes that “it is difficult to satisfactorily manufacture effective optical polarizers extending over an entire region of 400 nm to 700 nm from silver halide glass.”
As described above, a glass polarizer that is based on a stable manufacturing technology industrially widely applicable, is applicable to projection-type liquid crystal displays, and supports visible light does not exist.    Patent Document 1: Japanese Patent Application Laid-Open No.    Patent Document 2: Japanese Patent Application Laid-Open No.    Patent Document 3: Japanese Patent Application Laid-Open No.    Patent Document 4: U.S. Pat. No. 4,479,819    Patent Document 5: Japanese Patent No. 1618477    Patent Document 6: Japanese Patent No. 2740601    Patent Document 7: Japanese Patent No. 2885655    Patent Document 8: Japanese Patent Application Laid-Open No.    Patent Document 9: Japanese Examined Application Publication No. 2-40619    Patent Document 10: Japanese Patent No. 2628014    Patent Document 11: Japanese Patent No. 3549198    Non-Patent Document 1: N. Nishida, “Big-Screen Display (Series, Advanced Display Technology 7)”, Kyoritsu Shuppan, Tokyo, 2002    Non-Patent Document 2: S. Link and M. A. El-Sayed, J. Phys. Chem. B103 (1999), pp. 8410-8426    Non-Patent Document 3: K. Suzuki, Kogyo Zairyo Vol. 52, No. 12, pp. 102-107